I'd have to say the more gears, and the wider range, the better. But it's also going to be more expensive, and the improvement is not necessarily worth the additional cost.

I do most of my riding on a folding bike with an 8 speed hub, but I also ride 3 speeds quite a bit. I like them both; but the way I ride them is totally different: with the 8 speed I pretty much ride with the same cadence all the time (and it's fast), while with the 3 speed I have to vary it a lot more. If the lady is comfortable with a one speed hub, an eight speed will be a pretty radical change. In my opinion it's only worth it if she really wants to make a radical change in her riding style.